The season with the longest days and the highest average temperatures, summer astronomically begins on the summer solstice (when the tilt of the Earth's axis is closest to the Sun) and ends on the equinox (when the tilt of the Earth's axis is oriented neither from nor to the Sun causing the Sun to be located vertically above a point on the equator). The summer solstice occurs between June 20 and June 23 in the Northern Hemisphere and between December 20 and December 23 in the Southern Hemisphere. The autumnal equinox ending summer occurs between September 21 and September 24 in the Northern Hemisphere and March 21 or 22 in the Southern Hemisphere.

Meteorological summer is the season having the longest days and the highest temperatures. Summer extends for the whole months of months of June, July and August in the Northern Hemisphere and the whole months of December, January and February in the Southern Hemisphere. In other words, New York City experiences summer in July and Buenos Aires in January.

The highest recorded summer temperature was 136 F at Al' Aziziyah, Libya on September 13, 1922. The highest summer temperature recorded in the Southern Hemisphere was 123 F at Oodnadatta, Australia on February 1, 1960.

Meteorological summer is used to define the season in Australia, Denmark and the former USSR. It is also used in the United Kingdom, where summer is thought of as extending from mid-May to mid-August. In Ireland, summer begins on May 1 and ends July 31 by the Irish calendar. In southern and southeast Asia, where the monsoon occurs, summer is more generally defined as lasting from March to May/early June, their warmest time of the year, ending with the onset of the monsoon rains.

The astronomical definition of summer is more popular in the United States. However, the term summer season is considered to begin at the Memorial Day weekend (the last Monday in May) and end on Labor Day weekend (first Monday in September)..

In ancient China, summer was known as Red Bird.